Electrical surge arrestor having fail-safe properties

ABSTRACT

An electrical surge arrestor having fail-safe properties and providing breakdown at a predetermined region within a sealed structure. The arrestor comprises a pair of axially symmetric electrically and thermally conductive electrodes having integral terminal portions and discharge portions. The discharge portions are disposed coaxially in spaced confronting or nesting relationship within a coaxial ceramic insulative housing to provide a discharge gap at a selected area to form a sealed enclosure. The discharge gap is configured so that the discharge portions of the electrodes will fuse in response to the presence of a predetermined overload condition. The operating lifetime of the surge arrestor may be lengthened by shading an annular inner surface portion of the insulating housing from the discharge gap in order to minimize the deposite thereon of conductive material discharged from the electrodes during breakdown. The electrodes and insulating housing are formed of materials having substantially similar thermal expansion coefficients for providing a metal to ceramic seal resistant to thermal shock.

United States Patent Hill et al.

[451 Apr. 15, 1975 1 ELECTRICAL SURGE ARRESTOR HAVING FAIL-SAFEPROPERTIES [75] Inventors: John Hill, Bickley; Henryk Turczanski,Beckenham; Frederick Henry Wallis, Tunbridge Wells, all of England [73]Assignee: Comtelco (U.K.) Limited, I Tonbridge Kent, England [22] Filed:Dec. 11, 1973 [21] Appl. No.: 423,744

[30] Foreign Application Priority Data May 31, 1973 United Kingdom25961/73 [52] US. Cl. 313/267; 313/DIG. 5; 313/357 337 33 [51] Int. Cl.HOlj 1/94; HOlk 79/00 [58] Field of Search 313/267, DIG. 5, 357, 231.1,313/2312; 317/62; 315/36; 337/28-33 [56] References Cited UNITED STATESPATENTS 745,427 12/1903 Emonds 313/267 1,406,852 2/1922 Hendricks 317/621,406,858 2/1922 Hendricks 317/62 3,289,027 11/1966 Jones 317/613,373,308 3/1968 Perrin 313/357 3,408,525 10/1968 'Bahr 315/36 3,564,4732/1971 Kawiecki 337/28 3,649,874 3/1972 Peche 3l3/DIG. 5 3,651,3803/1972 Peche et a1. 313/214 X FOREIGN PATENTS OR APPLICATIONS 714,13910/1941 Germany 977,529 11/1966 Germany ..3l5/36 PrimaryExaminer--Alfred E. Smith Assistant Examiner-Wm. H. Punter Attorney,Agent, or FirmWeingarten, Maxham & Schurgin [57] ABSTRACT An electricalsurge arrestor having fail-safe properties and providing breakdown at apredetermined region within a sealed structure. The arrestor comprises apair of axially symmetric electrically and thermally conductiveelectrodes having integral terminal portions and discharge portions. Thedischarge portions are disposed coaxially in spaced confronting ornesting relationship within a coaxial ceramic insulative housing toprovide a discharge gap at a selected area to form a sealed enclosure.The discharge gap is config- .ured so that the discharge portions of theelectrodes will fuse in response to the presence of a predeterminedoverload condition. The operating lifetime of the surge arrestor may belengthened by shading an annular inner surface portion of the insulatinghousing from the discharge gap in order to minimize the deposite thereonof conductive material discharged from the electrodes during breakdown.The electrodes and insulating housing are formed of materials havingsubstantially similar thermal expansion coefficients for providing ametal to ceramic seal resistant to thermal shock.

18 Claims, 8 Drawing Figures ELECTRICAL SURGE ARRESTOR HAVING FAIL-SAFEPROPERTIES FIELD OF THE INVENTION This invention relates to electricalsurge arrestors and more particularly to'an arrestor having electrodesprovided within a sealed enclosure and providing failsafe operation inthe presence of a predetermined overload condition.

BACKGROUND OF THE INVENTION Surge arrestors are known in the art forproviding an electrical discharge path in the presence of predeterminedovervoltages. such as can occur from a lightning stroke. or currentsurges from other causes. For many purposes it is intended that thearrestor provide failsafe operation; that is. the arrestor must providea permanent short circuit discharge path in the presence of apredetermined overload;

One prior art arrestor is shown in US. Pat. No. 3.649.874 in which aninner (male) cylindrical electrode is disposed within a surroundingcoaxial outer (female) cylindrical electrode. The electrodes aremaintained in predetermined spaced relationship by means of a ceramicring. the area along the surface of the ceramic ring between theelectrodes being shielded to prevent deposition of evaporated conductivematerial thereon which could affect performance of the device. The maleelectrode is formed as a solid cylinder which is attached at one end toan end cap serving as one terminal of the device. the external surfaceof the female electrode constituting the other terminal. The end cap andthe female electrode are bonded to respective opposite ends of theceramic ring by means of compression seals employing glass as thesealing medium. Four separate parts are employed in the construction ofthe arrestor. and it is necessary to provide alignment between the maleelectrode and the end cap and in turn between the end cap and the femaleelectrode during manufacture in order to provide proper alignmentbetween the contact portions of the arrestor.

Another prior art arrestor is shown in US. Pat. No. 3.651.380 havingcontact portions comprising inner and outer cylinders and in which thethermal characteristics and dimensions of coaxial inner and outercylindrical electrodes are selected to ensure melting and bending of theinner electrode only thereby to provide physical contact therebetweenupon the application of a predetermined over-load to the arrestor. Therespective electrodes are attached to thin metal wall sections which inturn are sealed onto a ceramic ring to form a vacuum-tight housing. Thearrestor is formed of five separate pieces which must be separatelyattached whilst being maintained in alignment in order to provide anarrestor having contact portions having the desired alignment.

US. Pat. No. 3.454.81 1 shows a non-fail-safe surge arrestor in whichthe electrodes are separated by an annular ceramic housing. The areas ofthe inner surface of the ceramic housing adjacent the respectiveelectrodes are shielded from the deposit of sputtered conductivematerial by a series of setbacks on the ceramic housing and steps on theelectrodes. Respective contact portions of the electrodes are formed byaxially centered confronting. spaced. flat raised portions thereof;Contact portions of the electrodes are coated with carbon to reducepitting and to lower the arc discharge voltage thereacross.

Furthermore. the arrestor relies upon the sputtered material depositedupon the ceramic walls to assist initiation of a discharge and alsorefers to the statistical probability of sputtering activity occurringin the vicinity of the setbacks thus giving rise to the need for twosuch setbacks in order to ensure long operational life.

Although-the first two examples of prior art are ofthe fail-safevariety. they are designed so that fusion of the outer electrode doesnot occur in the overload condition in order to avoid perforationthereof. the short circuit that does occur is as a result of the tip ofthe inner electrode melting and forming a bead which touches the outerelectrode without actually fusing therewith.

The last example is not designed to short circuit upon overload andtherefore does not fall within the category of the invention.

The prior art arrestors discussed hereinabove display a complexity inconstruction which requires additional assembly steps to provide thedesired alignment between the electrodes. Misalignment which can occurduring the manufacture of such prior art arrestors can result inmalfunctioning arrestors and reduce the yield of the manufacturingprocess. Furthermore. the relative complex assembly and alignment stepscan increase the cost of the arrestor.

SUMMARY OF THE INVENTION Aims of the invention are to provide surgearrestors in which: one electrode fuses with the other upon occurrenceof a predetermined overload: there is a tendency for sputtered electrodematerial to be deposited upon a defined zone of the housing wall insteadof gen-- erally overall; the components are self-aligning in assemblyand of such design and material that thermal stresses therebetween areavoided.

It is proposed therefore to provide a surge arrestor in which electricaldischarge is caused to, occur at a precisely defined region within asealed gas tight enclosure and which in the presence of a predeterminedoverload condition causes fusion of 'one confronting electrode with theother in the defined region to thereby provide fail-safe operation. Suchan arrestor comprises a pair of solid axially symmetric conductiveelectrodes having integral discharge surfaces in nesting or confrontingarrangement and each having a thermally mas- 4 sive and integrallyformed terminal portion at one end thereof. The discharge surfaces aredisposed in confronting spaced relation thus defining a predetermineddischarge gap therebetween. The electrodes are sealed within and inintimate engagement with an electrically insulative housing to provideagas tight enclosure.

Accordingly the invention provides an electrical surge arrestorcomprising:

axially symmetric first and second solid. electrically conductive.electrodes of fusible material each formed with a terminal portion atone end thereof and a cylindrical portion extending co-axially from saidterminal portion. terminating in a discharge surface;

axially symmetric insulative housing adapted to receive in hermeticallysealed engagement said first and second electrodes thereby forming asealed chamber. said first and second electrodes being held by saidhousing in axial alignment one with the other and with said respectivecylindrical portions extending towards each other thus to define adischarge gap between the juxtaposed respective discharge surfaces. theshape of said respective discharge surfaces and the material of saidelectrodes being such that discharge occurs therebetween when anovervoltage occurs and that. in the presence of a predetermined overloadi.e. a function of voltage and time. the cylindrical portions of saidelectrodes will melt and fuse together.

The materials from which the insulative housing and electrodes areformed have matching coefficients of thermal expansion such materialsbeing. in one aspect of the invention. a ceramic material for theinsulative housing and a nickel-iron alloy for the electrodes. theceramic material containing a major proportion of alumina thenickel-iron alloy containing between 40 and 52 percent nickel.

The twodischarge surfaces may be mutually parallel i.e. providing adischarge gap of constant magnitude. but in practice it has been foundadvantageous to so shape the discharge surfaces that the discharge isencouraged to predominate in a zone which is situate between saiddischarge surfaces and of smaller volume than that of the discharge gapdefined by said discharge surfaces. For example. the discharge gap maybe of greatest magnitude at the radially outermost edge of the dischargesurfaces decreasing smoothly in magnitude with decreasing radius towardsthe central and mutual axis of the electrodes. Such zonal concentrationof the discharge between the electrodes is believed to assist in thefusion of one electrode with the other in an overload condition.

Accordingly. the discharge surface of said first electrode may besubstantially convex in configuration whilst that of said secondelectrode may be substantially planar. both surfaces being symmetricalabout the central axis and having radially outermost edges which lie inmutually parallel planes. Furthermore. the discharge surface of'saidsecond electrode may be substantially concave in configuration insteadof planar. and the discharge surfaces may be either both of spericalradius or conical in shape. The depth of the concave surface may be suchthat the discharge gap is disposed within the second electrode.

Further according to the invention, the radially outermost edges of thedischarge surfaces of said first and second electrodes may lie in planesparallel with the central axis and disposed on either side thereof. Atleast one of the discharge surfaces may be shaped so as t'o'provide adischarge gap of least and constant magnitude across and along thecentral axis.

So that the possibility is reduced 'ofa short-circuit occurring betweenthe electrodes due to deposition of sputtered electrode material uponthe internal walls of the insulative housing. the housing may beprovided with at least one annular surface facing away from saiddischarge gap. Such an annular surface may conveniently be provided witha coaxial annular groove so that a lip is created which shields thegroove surface from deposition of electrode material. The provision ofsuch a groove permits the use of a single annular surface only.especially when the surface is situated adjacent the terminal portion ofsaid second electrode when the electrode is provided with a concavedischarge surface. i.e. a discharge gap which tends to direct sputteredmaterial away from the terminal portion of said second electrode. Inpractice it has been found that a deep concave discharge surface. withsuit-ably nesting convex discharge surface. directs sputtered materialaway from the terminal portion of said second electrode sufficientlywell to obviate the need for any form of annular surface. i.e. setback.

The terminal portions of said first and second electrodes may be eachprovided with a cylindrical location surface and said insulative housingis recessed at opposite ends thereof to receive and hold the electrodesin the desired axial alignment. The electrodes may then be bonded to thehousing by known ceramic to metal bonding techniques such as. forexample: using titanium-cored silver solder in a neutral or reducingatmosphere; pre-metallising the ceramic and then brazing the electrodesthereto: using glass as an intermediate material.

Surge arrestors according to the invention are preferably filled with anatmosphere of ionised gas. or gas of low ionisation potential so thatthe breakdown potential threshold may be pre-determined.

BRIEF DESCRIPTION OF THE DRAWING The invention will be more fullyunderstood from the following described examples. with reference to theaccompanying drawings. wherein:

FlG.- 1 is a sectional elevation view of a surge arrestor provided withconical discharge surfaces:

HO. 2 is a partially cutaway sectional view of the discharge portions ofa surge arrestor such as that illustrated in FIG. 1;

FIG. 3 is a sectional elevation view of another surge arrestorhaving'discharge surfaces of sperical radius:

FIG. 4 illustrates in sectional elevation still another surge arrestorhaving one planar and one convex discharge surface:

FIG. 5 is a sectional elevation view of still another surge arrestor inwhich the use of setbacks is avoided;

FIGS. 6 and 7 show in section yet a further example of surge arrestorhaving discharge surfaces aligned parallel with the-central axis of thedevice: and

HO. 8 illustrates in sectional elevation an arrestor of the typeillustrated in FIG. 1 but having only one setback which is grooved toprovide improved shielding.

DESCRlPTlON OF THE PREFERRED EMBODlMENTS An electrical surge arrestorconstructed according to the invention is shown in typical embodiment inHO. 1 and comprises male and female electrodes 10 and 12 disposedco-axially within an electrically insulative ceramic housing 14. Maleelectrode 10 is a solid. axially symmetric. integral member and includesa generally cylindrical terminal portion 16, flange 18, a firstcylindrical portion 20. a narrower second cylindrical portion 22, andtruncated conical section 25 which forms the discharge surface thereof.Female electrode 12 is an axially symmetric integral member comprising aterminal portion 24. a flange 26. a first cylindrical portion 28 and asecond cylindrical portion 29. Terminal portions 24 and 16 may be ofidentical configuration and first cylindrical portions 28 and 20 willgenerally be of identical diameter for a purpose which will'hereinafterbe described. An axially symmetric cavity 30 is formed in cylindricalportions 29 having its opening at the inward facing end thereof toprovide a discharge surface for the female electrode. In the embodimentof FIG. 1, cavity 30 includes a truncated conical section 32 and agenerally cylindrical'section 34. The exact configuration of cylindricalsection 34 does not affect the performance of the surge arrestor and isthe result of employing conventional electrode fabrication techniques.

lnsulative housing 14 is of hollow. generally cylindrical configurationand includes annular recesses or setbacks 40 and 42 at the inner surfaceof the respective opposite ends thereof. Male and female electrodes and12 are mounted coaxially onto respective opposite ends of insulativehousing 14. the respectivecylindrical portions and 28 in intimateengagement with the inner surfaces 44 and 46 of insulative housing 14 atsetbacks 40 and 42. to preserve the desired alignment of the dischargeportions thereof. hi the embodiment of the arrestor illustrated in FIG.I. the discharge gap between the electrodes is defined by confrontingnonuniformly spaced surfaces of conical section and cavity 30 which. aspreviously noted are arranged in axially symmetric nesting relationship.the gap therebetween being greatest at the outermost radius and leastadjacent the central axis. FIG. 2 illustrates this nonuniformityclearly. in operation a discharge occurring between edge 23 and surface32.

The electrodes are formed of a nickel-iron alloy typically containing-52 percent nickel. while the insulative housing is formed typically of95 percent alumina. The thermal expansion coefficients of thenickel-iron electrodes and the insulative housing are substantiallysimilar to provide matched expansion during the often extremetemperature variations encountered during operation. Respectiveelectrodes are hermetically sealed at their respective flange portions18 and 26 to insulative housing 14 by brazing the electrodes ontometalized ends of the ceramic insulative housing. and which may beeasily accomplished. for example. in a furnace. Alternatively theelectrodes may be sealed to the insulative housing by means of ametal-glassceramic seal. By employing either seal embodiment. a seal isprovided which will not fail during breakdown and during overload untilfusion of the electrodes occurs. The flanges l8'and 26 are relativelythin and thus are somewhat flexible to withstand thermal shock withoutloss of hermeticity.

An atmosphere of argon may be provided within the sealed arrestor oralternatively a radioactive gas such as argon having a trace of krypton85 may be employed to provide an ionized atmosphere for enhancedelectrical discharge. As a further alternative. a radioactive paste orother material may be deposited within the arrestor enclosure typicallyat section 34 to provide the desired ionized atmosphere.

An alternative embodiment of the invention is illustrated in FIG. 3wherein the discharge portions of the electrodes comprise concave andconvex spherical sections and 62. disposed in confronting spaced andnested arrangement to provide a discharge gap 64 therebetween. Theconcave spherical cavity having a greater radius than the radius of theconvex section to provide a tapered discharge gap.

In a further embodiment of the invention as shown in FIG. 4, thedischarge portions of the respective electrodes are confronting flat endsurface 66 and slightly conical surface 68 of respective cylinders 70and 72 providing a discharge gap 74 across their confronting areas.preferential discharge occurring between peak 67 and surface 66.

- One other alternative embodiment ofa surge arrestor is shown in FIG. 5in which the insulative housing 76 of the arrestor is formed withoutsetbacks. The respective conical discharge portions 78 and 80 of themale and female electrodes 82 and 84 are nested one within the other todefine a discharge gap lying substantially within the female electrode.

In the operation of the surge arrestor of the invention. electricaldischarge. otherwise termed breakdown". will take place across adischarge gap in response to the application of a predeterminedpotential across the arrestor. The arrestor will continue to dischargeuntil the potential needed to sustain discharge is no longer exceeded.Should. however. the discharge potential of the arrestor continue to beexceeded for a predetermined length of time or if an overload potentialof sufficient magnitude is applied across the arrestor. the heatgenerated by current flow through the electrodes causes the electrodesto melt and fuse together at their discharge surfaces to form a highlyconductive electrical path across the arrestor. effectively shortcircuiting the overload potential to ground.

During normal breakdown across the discharge gap. electrode material issputtered and evaporated from the discharge portions of the electrodesinto the enclosure. If a substantial amount of such conductive materialis deposited onto the inner wall of the insulative housing an electricalpathway maybe formed between the electrodes along the inner wall of theinsulative housing which can cause short circuiting of the surgearrestor or otherwise materially affect its breakdown characteristics.Edges 86 and 88 defined by respective setbacks 40 and 42 enable surfaceportions 90 and 92 of the wall which lie within the setback to be shadedfrom direct sputtering thereon of conductive material from the regionofdischarge gap. Although this shading does not absolutely prevent thedeposition of conductive material onto the annular areas. it minimizesthe amount of sputtered material deposited thereon thus delaying theestablishment of an electrical pathway between electrodes along theinner wall of the insulative housing. As a result. the operativelifetime of the surge arrestor is lengthened.

In the embodiment of the invention illustrated in FIG. 5 in whichinsulative housing 76 includes no setbacks. edge- 94. defined by concavedischarge portion 80. in the female electrode provides a limited amountof shading from the deposit of sputtered material on annular surface96.0f the inner wall of insulative housing 76, the amount of shadingdecreasing with distance from the female electrode. Edge 98, defined byconical section 78 and the inner cylindrical portion of male electrode82 similarly provides limited shading from deposit of sputtered materialonto an annular surface 97 surrounding the male electrode. the amount ofshading decreasing in proportion to the distance from first cylindricalsection 100. Although this shading does not absolutely prevent thedeposition of conductive material onto any portion of the inner wall ofinsulative housing 76. it minimizes the amount of sputtered materialdeposited thereon and thus. similarly to the embodiment of the arrestoremploying setbacks. delays the establishment of an electrical pathwaybetween electrodes along the wall and lengthens the operative lifetimeof the arrestor.

Referring to FIGS. 6 and 7, the surge arrestor depicted has electrodes102. 104, having respective discharge surfaces 106 and 108 facing oneanother in spaced axial alignment. surface 108 being ridged to provide apreferential discharge path between ridge 110 and surface 106. End gaps112 and 1 14 are greater than the gap between ridge 110 and surface 106.

The arrestor depicted in FIG. 8 has electrodes 116 and 118 of the typeshown in FIG. 1. having a discharge gap 120 therebetween. and bondedinto ceramic housing 122 being inserted into respective recesses I24and. 126 thereof. A single recess or setback 128 is provided which hasan annular lip 132 which shields an annular groove 130 from depositionof sputtered metal. Such a' groove and lip affords improved shieldingand a longer operational life than a plain recess. such as 42 of FIG. 6for example. The provision of a grooved recess obviates the desirabilityof two plain recesses and is preferably situated adjacent the terminalof whichever electrode bears a concave discharge surface.

The surge arrestor of the invention may be employed with eitherelectrode connected to ground or a reference potential without affectingthe breakdown or failsafe properties of the arrestor. While theinvention has been described in typical embodiment. it will beappreciated that the novel apparatus may be constructed in a variety ofembodiments in addition to those exemplified hereinabove. the provisionof electrodes having mutually parallel discharge surfaces of conical.planar or other suitable shape being but one such variation. Accordinglyit is not desired to limit the scope of the invention by what has beenparticularly shown: the invention is limited only by the claims whichfollow.

What is claimed is:

1. An electrical surge arrestor comprising:

first and second. axially symmetric electrical discharge electrodes.each formed of a unitary solid block of the same material consistencythroughout. each block having an electrical discharge surface formed inone end face thereof. a peripheral flange extending transversely of thecentral axis of the block. a location portion contiguous with one sideof said flange and extending therefrom along said axis and a terminalportion contiguous with the opposite side of said flange and extendingtherefrom along said axis; and an insulative sleeve of uniform internalcross-section at least over a major portion of its length and into eachend of which said electrodes are respectively received; said peripheralflange of each electrode being seated against and hermetically sealed toa respective end face of said sleeve to form a sealed chamber and saidlocation portion of each electrode being fitted in a respective end ofsaid sleeve to align the electrodes along a common axis centrally withinsaid sealed chamber: said electrodes being dimensioned to provide anannular gap between the electrodes and theconfronting inner surface ofsaid sleeve and to define a discharge gap between confronting dischargesurfaces of said electrodes: the discharge surfaces of said electrodesbeing shaped to provide relative male and female confronting surfaces:said electrodes and said sleeve being constructed of materials havingsubstantially the same coefficient a of thermal expansion. I 2'. A surgearrestor as in claim 1 wherein said insulative housing is formed ofceramic material containing a major proportion of alumina and thematerial of which said first and second electrodes areformed is a 8nickel-iron alloy containing between 40 and 52 percent nickel.

3. A surge arrestor according to claim 1 provided with an atmosphere ofionized gas within the sealed chamber to thereby provide a selectedbreakdown potential threshold across said discharge gap.

4. A surge arrestor as in claim 1 wherein said insulative sleeve isprovided with at least one annular surface faking away from saiddischarge gap.

5. A surge arrestor as in claim 1 wherein the shapes of said dischargesurfaces are such that the discharge gap is of greatest magnitude at theradially outermost edge of said surfaces and decreases smoothly inmagnitude towards the central and mutual axis of said electrodes.

6. A surge arrestor as in claim 5 wherein the .discharge surface of saidfirst electrode is substantially convex in configuration and thedischarge surface of said second electrode is substantially planar. bothdischarge surfaces being symmetrical about the central axis andtheradially outermost edges of said discharge surfaces lying in mutuallyparallel planes.

7. A surge arrestor as in claim 5 wherein the discharge surface of saidfirst electrode is of substantially convex configuration and thedischarge surface of said second electrode is of substantially concaveconfiguration. both discharge surfaces being symmetrical about thecentr'al axis. the radially outermost edges of said discharge surfaceslying in mutually parallel planes.

8. A surge arrestor as in claim 7 wherein both discharge surfaces ofsaid first and second electrodes are of substantially conicalconfiguration, one being nested within the other.

9. A surge arrestor as in claim 7 wherein the dis charge gap is disposedsubstantially within said second electrode.

10. A surge arrestor as in claim 5 wherein the radially outermost edgesof the discharge surfaces of said first and second electrodes lie inplanes parallel with the central axis and disposed on either sidethereof.

11. A surge arrestor as in claim 10 wherein at least one of saiddischarge surfaces is shaped so as to provide atgap of least andconstant magnitude across and along the central axis.

12. A surge arrestor as in claim 1 wherein the discharge surfaces ofsaid first and second electrodes are shaped to provide a gaptherebetween which narrows at least along one .transverse axial linetoward the aligned central axis of said electrodes, such that dischargeis encouraged to predominate in the region of said narrowed gap.

13. A surge arrestor as in claim 1 wherein the discharge surfaces ofsaid first and second electrodes are symmetrical about the central axisthereof and have radially outermost edges lying in mutually parallelplanes. said discharge surfaces being shaped to provide a gaptherebetween which tapers toward said central axis such that dischargeis encouraged to predominate at the central axis region of said gap.

14. A surge arrestor as in claim 1 wherein said location portion of eachof said first and second electrodes has a cylindrical location surfaceconforming to the confronting surface of the respective end of saidsleeve. said location surface of each electrode being fitted in arespective end of said sleeve.

15. A surge. arrestor as in claim 8 wherein the electrode having saidconical discharge surface with respect to which the other conicaldischarge surface is nested includes a coaxial annular groove contiguouswith the outermost edge of said conical discharge surface and saidperipheral flange and confronting said chamber and a portion of theinner wall of said sleeve.

16. A surge arrestor as in claim 1 wherein said sleeve includes at leastone annular groove provided in the innerwall thereof and having anannular lip operative to shield said groove from deposition of sputteredmetal present during a discharge across said gap.

17. A surge arrestor as in claim 1 wherein said sleeve includes anannular groove provided in at least one end LII

1. An electrical surge arrestor comprising: first and second axiallysymmetric electrical discharge electrodes, each formed of a unitarysolid block of the same material consistency throughout, each blockhaving an electrical discharge surface formed in one end face thereof, aperipheral flange extending transversely of the central axis of theblock, a location portion contiguous with one side of said flange andextending therefrom along said axis and a terminal portion contiguouswith the opposite side of said flange and extending therefrom along saidaxis; and an insulative sleeve of uniform internal cross-section atleast over a major portion of its length and into each end of which saidelectrodes are respectively received; said peripheral flange of eachelectrode being seated against and hermetically sealed to a respectiveend face of said sleeve to form a sealed chamber and said locationportion of each electrode being fitted in a respective end of saidsleeve to align the electrodes along a common axis centrally within saidsealed chamber; said electrodes being dimensioned to provide an annulargap between the electrodes and the confronting inner surface of saidsleeve and to define a discharge gap between confronting dischargesurfaces of said electrodes; the discharge surfaces of said electrodesbeing shaped to provide relative male and female confronting surfaces;said electrodes and said sleeve being constructed of materials havingsubstantially the same coefficient of thermal expansion.
 2. A surgearrestor as in claim 1 wherein said insulative housing is formed ofceramic material containing a major proportion of alumina and thematerial of which said first and second electrodes are formed is anickel-iron alloy containing between 40 and 52 percent nickel.
 3. Asurge arrestor according to claim 1 provided with an atmosphere ofionized gas within the sealed chamber to thereby provide a selectedbreakdown potential threshold across said discharge gap.
 4. A surgearrestor as in claim 1 wherein said insulative sleeve is provided withat least one annular surface facing away from said discharge gap.
 5. Asurge arrestor as in claim 1 wherein the shapes of said dischargesurfaces are such that the discharge gap is of greatest magnitude at theradially outermost edge of said surfaces and decreases smoothly inmagnitude towards the central and mutual axis of said electrodes.
 6. Asurge arrestor as in claim 5 wherein the discharge surface of said firstelectrode is substantially convex in configuration and the dischargesurface of said second electrode is substantially planar, both dischargesurfaces being symmetrical about the central axis and the radiallyoutermost edges of said discharge surfaces lying in mutually parallelplanes.
 7. A surge arrestor as in claim 5 wherein the discharge surfaceof said first electrode is of substantially convex configuration and thedischarge surface of said second electrode is of substantially concaveconfiguration, both discharge surfaces being symmetrical about thecentral axis, the radially outermost edges of said discharge surfaceslying in mutually parallel planes.
 8. A surge arrestor as in claim 7wherein both discharge surfaces of said first and second electrodes areof substantially conical configuration, one being nested within theother.
 9. A surge arrestor as in claim 7 wherein the discharge gap isdisposed substantially within said second electrode.
 10. A surgearrestor as in claim 5 wherein the radially outermost edges of thedischarge surfaces of said first and second electrodes lie in planesparallel with the central axis and disposed on either side thereof. 11.A surge arrestor as in claim 10 wherein at least one of said dischargesurfaces is shaped so as to provide a gap of least and constantmagnitude across and along the central axis.
 12. A surge arrestor as inclaim 1 wherein the discharge surfaces of said first and secondelectrodes are shaped to provide a gap therebetween which narrows atleast along one transverse axial line toward the aligned central axis ofsaid electrodes, such that discharge is encouraged to predominate in theregion of said narrowed gap.
 13. A surge arrestor as in claim 1 whereinthe discharge surfaces of said first and second electrodes aresymmetrical about the central axis thereof and have radially outermostedges lying in mutually parallel planes, said discharge surfaces beingshaped to provide a gap therebetween which tapers toward said centralaxis such that discharge is encouraged to predominate at the centralaxis region of said gap.
 14. A surge arrestor as in claim 1 wherein saidlocation portion of each of said first and second electrodes has acylindrical location surface conforming to the confronting surface ofthe respective end of said sleeve, said location surface of eachelectrode being fitted in a respective end of said sleeve.
 15. A surgearrestor as in claim 8 wherein the electrode having said conicaldischarge surface with respect to which the other conical dischargesurface is nested includes a coaxial annular groove contiguous with theoutermost edge of said conical discharge surface and said peripheralflange and confronting said chamber and a portion of the inner wall ofsaid sleeve.
 16. A surge arrestor as in claim 1 wherein said sleeveincludes at least one annular groove provided in the inner wall thereofand having an annular lip operative to shield said groove fromdeposition of sputtered metal present during a discharge across saidgap.
 17. A surge arrestor as in claim 1 wherein said sleeve includes anannular groove provided in at least one end of said sleeve and having acircumferential surface of greater dimension than that of the innerdimension of said sleeve and an annular surface facing away from saiddischarge gap.
 18. A surge arrestor as in claim 1 wherein said sleeveincludes an annular groove provided in each end of said sleeve and eachhaving a circumferential surface of greater dimension than that of theinner dimension of said sleeve and an annular surface facing away fromsaid discharge gap.